The advances in battery technology cannot keep up with market demands. Effective solutions to extend or preserve battery lifetime and storage capacity are needed.
Batteries are electrochemical energy storage systems. The lifetime of a battery is limited by the aging process. A battery ages (loses its energy storage capacity) with use, and with time, even if not used. After the energy storage capacity of a battery has decreased below a threshold value it is said to have reached its end of life. Aging from use is called “life cycle aging”, while aging with time, is known as “calendar aging.” These two processes occur together, and their magnitudes depend on the battery's application and usage pattern. As such, one process may dominate.
A battery's operating conditions affect the aging process. Temperature and high charging voltages are some of the most relevant factors in aging. Exposing a battery to high temperatures and storing the battery in a full state-of-charge for an extended period may age a battery faster than charge-discharge cycling from use. Different battery types have different life cycle aging and calendar aging characteristics.
Lithium-ion batteries age when exposed to elevated temperatures and when stored in a fully charged (high voltage) state. A temperature above 30° C. (86° F.) is considered an elevated temperature. For most Lithium-ion (Li-ion) batteries, a cell voltage above 4.10 V/cell is considered a high voltage.
A laptop battery is usually exposed to an elevated temperature when operating. In normal use, a laptop battery is also usually fully charged and may rarely be disconnected from the charger. Some cell phone batteries have similar operating conditions. during their lives. It is common for a cell phone battery to lose much of its capacity during the first one or two years of its calendar life
A short description of how batteries operate is provided for convenience. Batteries contain chemically active materials and deliver energy through electrochemical reactions. When current is discharged from the battery, two concurrent phenomena occur:                i) the concentration of the active materials around the electrode drops forming a depletion region and generating a concentration gradient. This is known as the polarization effect. This concentration gradient acts as an internal resistance and reduces the charging efficiency of the battery;        ii) at the same time, the active materials move toward the depletion region due to a diffusion process resulting in a decreased gradient concentration and decreasing the polarization effect.        
Depletion typically occurs faster than diffusion. Both phenomena act concurrently to distribute the active materials. In the ideal case depletion takes place at a similar rate to the diffusion and the concentration of active material is at an ideal equilibrium point whereby the battery has maximum efficiency. In reality however, the equilibrium point settles at a different level. If diffusion constantly exceeds depletion effect, the battery delivers less energy than expected. Conversely, if the polarization effect overcomes the diffusion process, the battery will be discharged before the active materials are actually exhausted.
US patent application No. 2010/0164430 (Lu) shows that during discharging of a Lithium-ion battery cell (or cells), lithium ions accumulate on the cathode. Conversely, during charging, the lithium ions accumulate on the anode of the battery. The accumulation of ions during discharging generates a concentration gradient which compensates for the movement of ions during discharging. A gradient is also produced during the battery charging process. In an equivalent circuit model, the concentration gradients are modelled as internal resistances and have a negative effect on the battery efficiency during both charging and discharging cycles. If a battery has large internal resistance then a large portion of its stored energy will be dissipated internally by the internal resistance when it is discharging and will not be delivered to the load. Similarly the internal resistance will dissipate energy during charging making charging less efficient. A battery's storage capacity is therefore diminished by internal resistance.
It is known that battery capacity and lifetime can be improved by using a pulsed discharge current instead of a constant discharge current. This is due to a charge recovery process that takes place in the battery during the time between discharge pulses when the battery is not discharging, called the “rest time”.
It is also known that the battery lifetime and capacity significantly increases when a battery is pulse charged. Some studies also suggest that reversing the current for a short time during charging or discharging in a technique known as the ‘mode reversal’, positively affects the battery capacity and lifetime. For example when a short discharge pulse is applied prior to each charging pulse, this improves the charging process of the battery. When a short charging pulse is applied prior to each discharging pulse, this improves the battery recovery between discharging pulses and increases the battery storage capacity.
The battery's state of charge is directly related to the discharge current rate and decreased by it. However, if the current is interrupted, the battery's state of charge may recover and improve during the interruption. The recovery process is dependent on the duration of the interruption, the capacity of the battery, and the present state of charge of the battery. The recovery effect progressively decreases as the battery's state of charge decreases, until all the active materials are exhausted and the battery is depleted.
A simple known model describes the battery behavior during the discharge process and considers the recovery mechanism depending on the rest time only. Other battery models take into account the degradation of the recovery mechanism as the battery state-of-charge decreases.
Typically Li-ion batteries used in mobile applications last between one and four years during which interval they constantly lose the capacity to hold a charge for long periods of time. These battery systems are typically used in cell phones, cameras, tablets, battery packs, portable power tools and laptops due to their high energy density. The capacity loss is mainly due to increased internal resistance. Processes causing an increased internal resistance are more damaging when the battery charge state is close to full for longer periods. The internal cell resistance increases to a point where the battery pack can no longer deliver the stored energy irrespective of the fact that the battery indicates it is fully charged. In other words, a new battery when fully charged delivers its nominal charge. A used battery delivers less than its nominal charge.
Internal resistance typically increases for lithium-ion/lithium-polymer batteries with age and with each charge cycle. The aging speed is dependent on the working temperature and the battery state-of-charge. Most mobile devices constantly allow the battery to be at a fully-charged state while the device is plugged into a power adapter. Both these factors, namely, the constant fully-charged state of the battery and the elevated working temperatures contribute to a reduction of the battery life.